1. Field of the Invention
The present invention relates to improved gating for investment casting of metal wood golf club heads.
2. Description of Related Information
During the 1970's, advancements in investment casting technology led to the production of thin-walled, hollow golf clubs commonly referred to as "metal woods." The traditional "wood" golf club heads in use at the time were constructed from various types of wood or wood laminates, and required a solid or near-solid product. Hollow metal heads offered distinct advantages in playability over the traditional wooden clubs and, as a result, they have almost completely replaced traditional wooden clubs.
A metal wood golf club head typically comprises a main body having a hollow interior. The main body is characterized by a "face," the portion that strikes a golf ball, a "heel" and a "toe" on opposite ends of the face, an upper surface, or "crown," and a "ribbon" extending around and below the crown between the toe and the heel. The curvature of the face is characterized by its "bulge," or side-to-side curvature, and its "roll," or top-to-bottom curvature.
An orifice bounded by the lower portion of the face, heel, toe and ribbon opens into the hollow interior of the main body and is covered by a "sole plate." The sole plate is welded to the body at the orifice rim (hereinafter, the "weld rim"). The golf club head also has a hosel for connection to a club shaft, either extending from the exterior surface of the main body or incorporated within the body itself.
The primary production technique used to manufacture metal woods is investment or "lost wax" casting. For investment casting of ferrous and nonferrous golf club heads, static, gravity-driven, air-cooled investment casting, requiring no vacuum chamber, is typically employed.
According to the lost wax technique, tooling is constructed in the shape of the desired product and hot wax is injected into the tooling to form a three dimensional likeness or "pattern" in wax of the desired product. The wax pattern is affixed to a wax mold assembly, which is repeatedly dipped in a ceramic slurry and stuccoed with sand. This process "invests" or surrounds the wax assembly with a shell of hard ceramic.
When an adequately strong ceramic shell has been completed, the ceramic mold is fixed in an autoclave where the wax pattern is melted and removed via heat and steam, leaving a hollow ceramic impression of the entire assembly. The ceramic shell is then preheated, after which molten metal is poured therewithin, thus replacing and duplicating the original wax mold assembly. When the metal has cooled, the ceramic is shaken loose from the metal leaving a positive metal duplicate of the original wax part and assembly.
The molten metal is introduced into the mold assembly through an integral cup and is distributed to and through the ceramic mold through a manifold and channels. Gating channels are formed in the mold by incorporating "gates" into the wax pattern or assembly before the dipping process begins. The gates are wax bodies arranged in a desired flow pattern.
Gates may be created by the tooling or may be added by wax welding after the initial wax pattern is formed. Each gate extends from the wax pattern to a "runner bar," which is a positive wax manifold connecting the gates to the cup. When the wax gates and runner bar are melted and removed after dipping, a manifold and tributary channels remain, through which molten metal may travel.
In final processing, the unfinished metal head is cut free from the positive metal duplicate of the mold assembly, the metal which filled the gating channels is cut off, and the remaining material is ground and polished to form a finished golf club head.
Because the gating arrangement governs the distribution pattern and rate for molten metal as it is introduced, it also affects the cooling rate and the structure of the club head. In this regard, an even cooling pattern proceeding from the outside to the inside of the head produces a fine, even grain structure which is desirable for cosmetic and structural reasons.
A poor metal distribution profile can result in the "pitting" of a club head, weakening the part and requiring additional welding and polishing for cosmetic reasons. Pitting results from "shrink," which is associated with metal cooling. Shrink occurs where molten metal is trapped in a pocket, surrounded by cooler metal. Because the volume of metal decreases as it cools, the volume of the molten metal decreases. When there is no longer sufficient molten metal to fill the original pocket, a void or cavity can form and pitting can result.
In the past, gating for hollow metal wood golf club heads consisted of a solitary gate applied to the heel area of the club or, occasionally, to the toe area. Although this gating enjoyed relatively uncomplicated final processing, the use of a solitary gate restricted flow of metal into the product and could not ensure a tight grain or a good casting surface where used in conjunction with thinner heads.
Because solitary gates were not suited to increasing cosmetic quality and structural demands, secondary gates were added to the toe, crown and ribbon of the wax pattern to accomplish even distribution of metal flow and to control the solidification of the metal. As the quality race escalated, and the size of club heads increased (drivers especially), the number of gates required to maintain cosmetic and structural standards increased as well, being driven by the size and complexity of the club head being produced. In the past, a minimum of three gates and often as many as nine gates were required to meet quality standards.
While metal distribution control was accomplished to some extent through additional gating, penalties were seen in increasingly complex manufacturing. For example, gates on the toe, ribbon, and crown cannot be incorporated into a tool, but must be wax welded to the wax pattern. Because the gates must be cleaned and prepared before welding, and because of the substantial labor involved in welding the gates to the wax patterns, processing time and expense were significantly increased.
Significant labor was involved in ensuring an adequate seal, having no air pockets or undercuts, at the junctions between the gates and the runner bar or the wax pattern. This was necessary because undercuts can trap sand at the ceramic dipping stage. At the metal pouring stage, the sand particle can break off, and travel into the metal stream, resulting in an "inclusion." Inclusions affect aesthetics, processing time, and the strength of the club head.
At the dipping stage, additional gates to the toe, ribbon, and crown created at least two disadvantages. First, additional ceramic material was required to accommodate the extra surface area of the gates. Second, the additional gates aggravated a problem known as ceramic bridging, wherein ceramic material fills the area between two gates, leading to heat retention problems in the metal pouring stage.
The number and location of additional gates also complicated final processing, resulting in additional polishing, cutting, and other labor-intensive processing. For example, after the metal head is formed, additional gates allow ceramic material to cling more readily to the metal part, impeding its removal. In addition, when club heads are removed from the assembly, it is customary to use a bandsaw to cut through the metal gates. Cutting through many gates in varied locations with a bandsaw is labor intensive, time consuming, expensive and dangerous.
Once the gates were cut off, the areas of the club head where the gates were located required two stages of abrasive grinding, the first to remove stock, the second to blend the gate to the shape of the surrounding contour. This was time consuming and expensive and required excellent polishing skills in blending. If the blending was not done properly it could leave flat spots or thin spots which could result in scrapping a part. The abrasive belts used in these operations are very expensive and represent a substantial cost in manufacturing metal wood golf club heads.
Another problem associated with gating methods employed in the past was the lack of consistency in face curvature and sole plate fit. In the past, the sole plate orifice was prone to deformation, resulting in variation from piece to piece and poor or inconsistent sole plate fit. Deformation of the face has also been problematic in the past. Tooling creates the initial curvature of the wax pattern face. But removing the core from the wax pattern creates a slight vacuum, which tends to pull the face inward and to reduce its curvature. Contraction associated with the cooling of the molten metal tends to aggravate this effect.
In the past, the face deformation problem was compounded during the metal cooling stage due to stress generated by contraction of the head and gates during the cooling of the metal. Variations in the number of gates and their location combined to make the consistency of the final curvature unpredictable.
Thus, there is a need for an apparatus for casting metal wood golf club heads that employs a gating system that is cut directly into tooling, that results in suitable metal distribution and cooling patterns within the golf club head, that reduces or eliminates the occurrence of undercuts, that controls face curvature and sole plate orifice deformation, that does not suffer adverse effects from ceramic bridging, and that simplifies processing of the wax pattern and the unfinished metal golf club head.